The present invention relates to the general field of variable-pitch vanes for a turbomachine, and more particularly to bushings for the guide pivots of such vanes.
The high pressure compressor of a gas turbine typically comprises a plurality of circular stages of vanes of orientation that can be adjusted so as to modify the flow characteristics of the gas, depending on the operating speeds of the turbomachine. These vanes are referred to as variable-pitch vanes.
Each variable-pitch vanes in a given stage has a control pivot at a radially-outer “head” end and a guide pivot at a radially-inner “foot” end. The control pivot passes through the stator casing of the turbomachine and co-operates with a control member. By acting on the control member, it is possible to modify the orientation of the vanes in the stage concerned. The guide pivot of each vane moves in a bushing that is mounted in a corresponding recess in an inner ring inside the turbomachine and centered on the longitudinal axis of the machine.
While assembling the vanes on the inner ring, it is important to ensure good centering of the guide pivot for the vanes. This centering is obtained by ensuring that the bushings and the recesses in the inner ring in which the bushings are mounted are accurately coaxial. The accuracy with which the vanes are centered must also be maintained regardless of the operating stage of the turbomachine. Unfortunately, techniques known in the prior art for mounting bushings in the inner ring do not include any particular arrangement for ensuring that the quality of centering is maintained. The centering of the vane guide pivot thus tends to become less good in operation, in particular when the bushings of the pivot support and the inner ring are made of materials that have different coefficient of thermal expansion.